linux-kernel - Re: [PATCH] ext4: fix marking group trimmed if all blocks not trimmed

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Message-ID: <20200807220147.GA7657@mit.edu>
Date:   Fri, 7 Aug 2020 18:01:47 -0400
From:   tytso@....edu
To:     Lazar Beloica <lazarbeloica@...il.com>
Cc:     linux-ext4@...r.kernel.org, stable@...r.kernel.org,
        lazar.beloica@...anix.com, boyu.mt@...bao.com,
        adilger.kernel@...ger.ca, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: fix marking group trimmed if all blocks not trimmed

On Mon, Aug 03, 2020 at 04:17:44PM +0000, Lazar Beloica wrote:
> When FTRIM is issued on a group, ext4 marks it as trimmed so another FTRIM
> on the same group has no effect. Ext4 marks group as trimmed if at least
> one block is trimmed, therefore it is possible that a group is marked as
> trimmed even if there are blocks in that group left untrimmed.
> 
> This patch marks group as trimmed only if there are no more blocks
> in that group to be trimmed.

This patch makes no sense; first of all, the changes below are *not*
in the function ext4_trim_extent(), but rather ext4_trim_all_free().
It appears that the diff is based off of v5.8-rc2, based on the index
c0a331e, but then I'm not sure how you generated the diff?

Secondly, ext4_trim_all_free(), which is where these two patch hunks
appear:

> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index c0a331e..130936b 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -5346,6 +5346,7 @@ static int ext4_trim_extent(struct super_block *sb, int start, int count,
>  {
>  	void *bitmap;
>  	ext4_grpblk_t next, count = 0, free_count = 0;
> +	ext4_fsblk_t max_blks = ext4_blocks_count(EXT4_SB(sb)->s_es);
>  	struct ext4_buddy e4b;
>  	int ret = 0;
>  
> @@ -5401,7 +5402,9 @@ static int ext4_trim_extent(struct super_block *sb, int start, int count,
>  
>  	if (!ret) {
>  		ret = count;
> -		EXT4_MB_GRP_SET_TRIMMED(e4b.bd_info);
> +		next = mb_find_next_bit(bitmap, max_blks, max + 1);
> +		if (next == max_blks)
> +			EXT4_MB_GRP_SET_TRIMMED(e4b.bd_info);
>  	}
>  out:
>  	ext4_unlock_group(sb, group);

The function send discards for blocks in a block group which are
freed.  So setting max_blks to be ext4_blocks_count() and then using
it as the limit to mb_find_next_bit() makes no sense.  First of all
next will never be equal to max_blks, since next is an offset relative
to the beginning of the block group, and max_blks is set number of
blocks in the entire file system.

Secondly, mb_find_next_bit is searching a bitmap, which is a single
file system block (e.g., 4k in a 4k block file system).  So if
max_blks is the the number of blocks in (for example) a 10TB file
system, this is going to potentially cause a kernel oops.

How, exactly did you test this patch?

	     	      	 	     	   - Ted